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Freewheeling notes

Page 18

All Staffa motors can be used in freewheeling applications. In all circumstances it is essential that the motor is unloaded (A and B ports connected together) and that the circuit is boosted.The required boost pressure will be dependent on speed and displacement. It should be noted that for ‘B’ series motors large flows will re-circulate around the motor. This will require a large re-circulating valve and consideration of circuit cooling as the motor will generate a braking torque. It is for these reasons that ‘C’ series motors are the preferred option for freewheeling applications. It is normal to select displacement codes 10, 05 or 00. Selecting the lowest available displacement of zero (00) will allow the motor shaft to be rotated at high speed without pumping fluid and with a minimum boost requirement. This will result in a minimum drive torque requirement for the freewheeling motor. Examples of the freewheeling feature on a winch are : dropping the load quickly in the case of an emergency and paying out cable. Consideration should be given when freewheeling such that the load does not drive the motor above its rated freewheeling speed.

Displacement selection

If the motor inlet/outlet pressure is below 3.5 bar, then a minimum 3.5 bar control pressure is required in order to ensure that the motor remains in minimum displacement. It should be noted that in the event of loss of control pressure, the motor will shift to its highest displacement, which could result in damage to the motor. When freewheeling with displacement codes: 00, 05 or 10, it can be difficult to generate a 3.5 bar pressure. In these circumstances it is necessary to feed the displacement change control circuit from a separate source thus ensuring a minimum control pressure of 3.5 bar. Under all operating conditions the control pressure port should be at least 2/3 of the motor inlet/outlet pressure ports.

Boost requirement

The required boost pressure is detailed on page 15. The actual required level will be determined by the expected maximum speed in maximum displacement during the overrunning condition. A maximum motor and control pressure of 17 bar at 1000 r.p.m. is stated in the bulletins, although for purposes of freewheeling it is better to maintain a minimum boost level that satisfies all motor operating conditions. The Staffa motor bulletin boost formulae does not apply to freewheeling displacements. High boost levels will increase motor losses at the conrod slipper interface and valve assembly, which will increase the motor operating temperature.The boost flow required should be sufficient to make-up circuit leakage loss and provide cooling for recirculating flow pressure drop.

Crankcase cooling

A crankcase flushing flow of up to 15 l.p.m. can be used to control and reduce the temperature rise of the motor during the freewheeling operation. This should not be necessary for motor speeds upto 1000 r.p.m. If operating at speeds above 1000 r.p.m., then consult KPM.